This invention relates to the preparation of diaryl carbonates by oxidative carbonylation. More particularly, it relates to the improvement of diaryl carbonate yield in the carbonylation reaction.
Diaryl carbonates are valuable intermediates for the preparation of polycarbonates by transesterification with bisphenols in the melt. This method of polycarbonate preparation has environmental advantages over methods which employ phosgene, a toxic gas, as a reagent and environmentally detrimental chlorinated aliphatic hydrocarbons such as methylene chloride as solvents.
Various methods for the preparation of diaryl carbonates by an oxidative carbonylation (hereinafter sometimes simply "carbonylation" for brevity) reaction of hydroxyaromatic compounds with carbon monoxide and oxygen have been disclosed. In general, the carbonylation reaction requires a rather complex catalyst. Reference is made, for example, to U.S. Pat. No. 4,187,242, in which the catalyst is a heavy Group VIII metal; i.e., a Group VIII metal having an atomic number of at least 44, said metals consisting of ruthenium, rhodium, palladium, osmium, iridium and platinum, or a complex thereof.
A further development in the carbonylation reaction, including the use of compounds of other metals such as lead or cerium as cocatalysts, is disclosed in various patents including U.S. Pat. No. 5,498,789. Also required according to that patent is the use of quaternary ammonium or phosphonium halides, as illustrated by tetra-n-butylammonium bromide, as part of the catalyst package.
The commercial viability of the carbonylation reaction would be greatly increased if a less expensive compound could be substituted for the quaternary ammonium or phosphonium halide. It has been discovered, however, that substitution of such compounds as sodium bromide normally results in the isolation of the desired diaryl carbonate in low or insignificant yield.
The production of carbonates may be improved by including a metal-based cocatalyst along with the heavy Group VIII metal catalyst. Although the identity of suitable metal-based cocatalysts will depend on specific reaction conditions including the identity of reactants and other members of the catalyst package, some general guidance can be found in U.S. Pat. Nos. 4,187,242 and 4,201,721.
In U.S. Pat. Nos. 5,543,547 and 5,726,340, the use of carbonylation catalyst systems including palladium or an analogous metal, various cocatalytic metals which may include cerium, lead or cobalt, and an alkali metal or quaternary ammonium bromide is disclosed. Japanese Kokai 10/316,627 discloses a similar process in which the cocatalyst is a manganese or lead compound and a carboxylic acid amide or alkylurea is also present.
The use of a specific amide, N-methylpyrrolidone (hereinafter sometimes "NMP"), in a system employing a cobalt cocatalyst and, as an organic cocatalyst, a terpyridine or the like is disclosed in U.S. Pat. No. 5,760,272. The sole disclosed function of the NMP is to improve the selectivity to the formation of diaryl carbonate, as opposed to by-products such as biphenols. Further study of this system has revealed that it affords no improvement in diaryl carbonate yield defined in terms of "turnover number"; i.e., the number of moles of diaryl carbonate formed per gram-atom of Group VIII catalytic metal present. This is contrary to the suggestion in the Japanese Kokai, which clearly teaches an improvement in yield.
It is of interest, therefore, to develop catalyst systems which include an inexpensive halide compound and which can efficiently produce diaryl carbonates.